1. Field of the Invention
The present invention relates to a power supply switching circuit for switching an output voltage between a voltage of a voltage regulator and a voltage of a battery.
2. Description of the Related Art
FIG. 9 is a circuit diagram illustrating a conventional power supply switching circuit.
The conventional power supply switching circuit includes a voltage detection circuit 903, an error amplifier circuit 904, a reference voltage circuit 908, resistors 906 and 907, PMOS transistors 901, 902, and 905, diodes 909 and 910, input terminals 911 and 912, an output terminal 913, and a ground terminal 100.
A voltage input to the input terminal 911 is represented by V1, and a voltage input to the input terminal 912 is represented by V2. When the voltage V1 increases to exceed a set voltage of the voltage detection circuit 903, an output of the output terminal 913 becomes a high voltage level. As a result, the PMOS transistor 905 is turned OFF, and the PMOS transistor 901, the resistors 906 and 907, the error amplifier circuit 904, and the PMOS transistor 902 operate as a voltage regulator so that a stabilized constant voltage is output from the output terminal 913.
A voltage appearing in a positive power source of the error amplifier circuit 904 is supplied from a connection point between the diodes 909 and 910, and is therefore a higher one of the voltage V1 and the voltage V2 regardless of a switch operation. Thus, the error amplifier circuit 904 operates stably regardless of the switch operation and a load fluctuation. On the other hand, when the voltage V1 of the input terminal 911 drops to invert the output of the output terminal of the voltage detection circuit 903 to a low voltage level, the PMOS transistor 905 is turned ON. The PMOS transistor 901 is turned OFF because the PMOS transistor 902 is turned ON to change a gate of the PMOS transistor 901 to the high voltage level. Consequently, the voltage V2 of the input terminal 912 is output to the output terminal 913 (see, for example, Japanese Patent Application Laid-open No. Hei 06-244697).
However, the conventional power supply switching circuit has the following problem.
When the voltage of the input terminal 911 (voltage V1) is low and when the PMOS transistor 901 of the voltage regulator is turned OFF, there is no channel in the PMOS transistor 901. In other words, immediately after the PMOS transistor 902 is turned OFF, the PMOS transistor 901 cannot supply a load current. Thus, if the power source is switched to the voltage regulator in the state where a load is connected to the output terminal 913, a voltage fluctuation such as undershoot occurs at the output terminal 913.